This invention relates generally to sprue bushings for injection molding machines and, more particularly, to internally-heated sprue bushings in which the sprue bushing casing and bushing core are integral and to the method of making such sprue bushings. In this regard, an important aspect of the present invention is directed to an internally heated bushing assembly which is especially suited to handle pressurized melt, which assembly includes a unitary bushing casing-core element having an integral outer side wall, end wall and elongate inner core having a melt running passage therethrough.
Sprue bushings are commonly used in injection molding to provide a heated flowpath for conveying molten plastic or melt between an injection molding machine manifold or nozzle and an injection mold cavity opening. Typically, these sprue bushings include a separate bushing casing portion which may house a heating element and a separate bushing core portion through which the melt flows. The bushing core is usually disposed in the center of the bushing casing and extends axially through the casing between the bushing inlet and an outlet end thereof with one end of the core projecting through a hole in the casing. Since the core and casing are separate, however, the core must be welded to or press fit to the bushing casing at the bushing outlet end.
A serious problem presented by this type of sprue bushing construction arises from a failure at the junction of the discharge end of the inner core and the outlet end of the bushing casing. These sprue bushings are prone to such failure, even when the discharge end of the core is welded to the bushing, since the junction therebetween is subjected to high shear stress induced by the injection and nozzle seating pressures. In particular, when such failure occurs, the high injection molding and nozzle seating pressures encountered by these sprue bushings in operation results in the melt being forced into the heater element causing a shorting between the heating element wires. As a consequence of this shorting, premature burnout of the heater element results, necessitating replacement of the sprue bushing.
The present invention avoids the above-mentioned shortcomings and is directed to a novel sprue bushing assembly in which the bushing casing and core are integral, being formed from a one piece metal blank. As such, these sprue bushing assemblies are characterized by a unique bushing casing-core element which includes an integral outer side wall, end wall and elongate inner core having a melt runner passage extending therethrough. In this manner, failure at the junction of the discharge end of the inner core and the outlet end of the bushing casing is avoided, thereby eliminating melt leakage and internal heating element contamination.
In a sprue bushing assembly incorporating the principles of the present invention, a single cylindrical metal blank is provided with an annular cavity axially extending within the blank to form an elongate inner core member which is spaced apart from the outer wall, or casing, of the bushing and which inner core axially extends through the cavity. The annular cavity contains at least one heating element in the form of a hollow preformed ceramic electrically non-conductive sleeve which encircles the inner core. Open spaces in the annular cavity between the heating sleeve, the bushing casing and core are filled with a powdered or particulate refractory material to form a heater-bushing blank assembly. When filled, the heater-bushing blank assembly is then swaged and reduced down to a preselected uniform diameter, thereby compressing the refractory material and preformed heating element into a unified mass which fills the annular cavity and which is free of all air voids. A head member containing conduction and temperature sensing wires to the heating element is then added to the top portion of the bushing-heater assembly and the cap then firmly welded to the bushing assembly. An opening is drilled through the bushing core to provide a melt flowpath. The outlet end of the bushing is then finished to form a final bushing outlet tip.
Accordingly, it is a general object of the present invention to provide a method of manufacturing an improved sprue bushing from a single metal blank wherein the inner core is integral with the bushing casing which eliminates melt leakage and heater element burnout.
Another object of the present invention is to provide an injection molding unitary sprue bushing in which the bushing core is integral with the bushing casing and which has a multi-stage heating element for supplying heat to different areas of the bushing core.
A further object of the present invention is to provide a unitary sprue bushing wherein the bushing includes an outer annular casing, an inner core axially extending through the bushing casing, a bushing outlet that is integral with the bushing casing and inner core, and an annular cavity encircling the inner core and separating the bushing casing from the inner core which cavity contains at least one performed non-electrically conductive sleeve.
A yet further object of the present invention is to provide a sprue bushing in which the inner core is integral with the bushing casing and in which the bushing heating element has at least one thermowell formed therein which is adapted to receive a removable thermocouple.
Another object of the present invention to provide a unitary sprue bushing having an inner core which is integral with the bushing casing, wherein the inner core and casing are separated by an annular cavity adapted to receive a heating element therein which encircles the inner core and fills a portion of the annular cavity, the remainder of the cavity being filled by a particulate refractory material.
A still another object of the present invention is to provide a sprue bushing for injection molding having an outer bushing casing, an inner core integral with the bushing casing and axially extending therethrough, the inner core having a melt runner passage axially extending through the bushing, the bushing further including an annular cavity disposed between the inner core and bushing casing, the cavity having two heating element means in the form of preformed sleeves axially aligned therein, each heating element means providing heat to a different portion of the inner core of the sprue bushing.
These and other objects, features and advantages of the present invention will be apparent from the following detailed description, taken in conjunction with the accompanying drawings wherein like reference numerals refer to like parts.